Conventionally, there is known a sheet forming device configured to extrude an amount of material kneaded by a kneading machine of a double tapered screw type acting as a supplying unit and to roll the extruded material into a sheet form, with a rolling unit having two rolls (see e.g. Patent Documents 1 and 2).
The sheet forming device of the above noted type includes a supplying unit, a storing unit and a rolling unit. In operation, an amount of material is extruded from the supplying unit to the storing unit, in which the extruded material is stored for a while. Thereafter, the material is transferred to the rolling unit to be formed into a sheet of material having a planned formed sheet thickness (see e.g. Patent Document 3).
Of the above, in the sheet forming device disclosed in Patent Document 1, between the supplying unit constituted of an extruding machine of two tapered screw type and the rolling unit comprised of two rolls, there is provided a storing unit for temporarily storing the amount of material supplied from the supplying unit. Then, this material stored in the storing unit is sent sequentially to the rolling unit to be formed into the sheet form.
In the meantime, the storing unit of the above-described sheet forming device is a so-called “open bank type” storing unit having an open top. With this type of sheet forming device, the device provides a difference absorbing function according to which when any difference has occurred between the receiving amount of the material from the supplying unit (the supplying amount by the supplying unit) and the feeding amount of the material fed to the rolling unit (the paying-out amount by the rolling unit), this difference is absorbed by increase/decrease in the amount of material stored inside the storing unit. On the other hand, however, in association with such variation in the supplying amount by the supplying unit, there tends to occur change in the pressure in the material being fed to the rolling unit, which change results in variation in the nipping (rolling) amount of the material in the rolling unit. This variation may result in such problems as disturbance in the shape of the formed sheet at its opposed ends and irregularity in the thickness thereof.
In an attempt to solve the above problem, as in the case of the sheet forming device disclosed in Patent Document 2, the art has proposed to detect the amount of material stored in the storing unit so that depending on this detected amount, the receiving amount of the material from the supplying unit may be controlled. However, with such controlling of the material stored in the storing unit alone, it has been proven difficult to eliminate the variation in the nipping amount due to the variation in the pressure of the material being fed to the rolling unit.
On the other hand, as described in Patent Document 3, the art has provided also the so-called “closed bank type” sheet forming device with the top of the storing unit thereof being closed. However, in the case of material having elasticity, there occurs a pressure distribution (irregularity of the magnitude of pressure according to the position) in the pressure applied to the elastic material stored in the storing unit. Hence, even if the receiving amount of the elastic material from the supplying unit is controlled based on detected value of the pressure applied to the elastic material at a particular position, it has been difficult to eliminate the variation in the nipping amount due to the variation in the pressure of the material to the rolling unit.
Also, in the case of the closed bank type sheet forming device, this type of device does not provide sufficiently such difference absorbing function for absorbing any difference which may have occurred between the receiving amount of the elastic material from the supplying unit and the feeding amount of the elastic material fed to the rolling unit. Therefore, if there develops a sharp buildup in the inside pressure of the storing unit due to e.g. response performance of the control mechanism for the supplying unit, this applies significant load to the constituting members of the supplying unit and/or the rolling unit, thus inviting damage to the device.
Moreover, in the case of the closed bank type sheet forming device, for effecting inspection, cleaning of its storing unit, the device needs to be disassembled, thus requiring much trouble.
Further, conventionally, the material charged into the supplying unit is kneaded in advance by a kneading machine. However, if there occurs a problem or deficiency in the kneading operation and/or inappropriateness of the kneading temperature, there may occur deficiency in the plasticization, which results in turn in formation of hard portions, which in turn leads to application of an excessive load to the rolling unit or failure to nip the material due to slippage of the material, thus leading to failure in rolling of the material in the rolling unit or clogging of the material in the rolling unit.
Moreover, such trouble in the sheet forming device may cause a trouble in a device operation-wise subsequent thereto, so that the entire sheet forming device needs to be temporarily stopped in its operation. Then, the material stored in the storing unit is hardened, so that when the operation is resumed, material clogging may occur in the rolling unit.
In the event of occurrence of such trouble as above, it was required that the operation of the entire device be stopped and the clogged material be removed from the storing unit.
That is, with these conventional solutions, the solutions required too much trouble and also, while the device is stopped, there occurs change in the viscosity of the material which change increases the viscosity, which makes the removal of the clogged material even more difficult to the disadvantage of the solution of the trouble.    Patent Document 1: Japanese Published Patent No. Hei. 2-42331    Patent Document 2: Japanese Patent Application “Kokai” No. Hei. 5-104607    Patent Document 3: Japanese Patent Application “Kokai” No. 2006-231582 (paragraph 0044).